The present invention relates to a communication system operated by means of a code division multiple access mode, and more particularly to a spread spectrum receiver using the code division multiple access (CDMA) mode for integrating two received signals from in-phase (I) and quadrature (Q) channels, and selecting the signal of the channel having the larger amplitude between the two integrated signals, thereby easily despreading and demodulating the signal.
Communication systems operated using the spread spectrum CDMA mode transmit digital data. By utilizing a mixer (or an exclusive OR gate) for mixing a relatively high frequency signal with a pseudo-noise code, a spread signal having an extremely wide bandwidth and a low spectral density is produced, and then the spread signal is transmitted during the general spread spectrum CDMA mode, in order to eliminate or decrease continuous wave (CW) and jamming signals. Thus, such a communication system is made suitable for the communications within a building. Also, the signal having low spectral density decreases the interference of several radio waves.
U.S. Pat. No. 5,121,407 discloses a spread spectrum receiver which has recently been developed. Here, the CW and jamming signals are eliminated or decreased by utilizing frequency shift unit without requiring automatic gain controller to receive and stably demodulating the signal formed and transmitted according to the above-described method, so that short delay multi-path problem within a building is solved, frequency and dynamic range which is a variable operational characteristic of a tuner are enlarged, and acquisition time of the signal is reduced.
Further, as shown in FIG. 1, a generalized spread spectrum receiver using the CDMA mode includes: first and second mixers 11 and 12 for down-converting a received spread signal, using local oscillator frequencies f.sub.L01 and f.sub.L02 ; a first RF (radio frequency) amplifier 13 for amplifying the frequency-converted signal; a voltage controlled oscillator 14 for producing a coherence clock; a frequency shift unit 24 for shifting the phase of the reference frequency (the coherence clock) by as much as 90.degree. (.pi./2); first and second low-pass filters 15 and 16 for eliminating two spread signal components of an I channel and a Q channel, respectively; third RF amplifiers 17 and 18 for amplifying each filtered signal; first and second analog-to-digital (A/D) converters 19 and 20 for respectively converting the amplified signals into digital signals; an absolute value detector 21 for obtaining absolute values of two signals of the I and Q channels, a peak detector 22 for detecting the peak of the absolute values; and a despreading demodulator 23 for despreading the selected signal to produce an original signal.
In the receiver of the conventional spread spectrum CDMA mode formed as the above, the peaks of the absolute values with respect to two signals are taken and then summed, thereby obtaining the signal value. In more detail, a spread signal transmitted from a transmitter is received, and the received signal is down-converted, using output frequencies f.sub.L01 and f.sub.L02 of the local oscillators in first and second mixers 11 and 12. Then, the signal is amplified in first RF amplifier 13, and mixed with a coherence signal f.sub.corh, thereby producing a signal of the I channel. Additionally, a signal of the Q channel is produced such that a signal f.sub.corh+90.degree. obtained by shifting the coherence signal f.sub.corh signal supplied from voltage controlled oscillator 14 by 90.degree. in frequency shift unit 24 is mixed with the signal supplied after being amplified in first RF amplifier 13. The two signals of the I and Q channels generated according to the foregoing method are passed and filtered through first and second low-pass filters 15 and 16, respectively, so that spread signal components are eliminated. Thereafter, the filtered signals are amplified, using second and third amplifiers 17 and 18. The absolute values of two signals of the I and Q channels which have been digitized via first and second A/D converters 19 and 20 are obtained in absolute value detector 21. Finally, the peaks of the absolute values are detected in peak detector 22, and then the peaks are summed, thereby obtaining the signal value.
Another method for processing a spread signal is the so called costas loop system, in which the difference component between the two signals of the I and Q channels obtained in the above manner is extracted. Then, the spread signal value is obtained from one channel between the I and Q channels, by supplying an error voltage to a voltage controlled oscillator which generates a coherence signal, using a phase-locked loop.
The method which obtains the signal value by taking and then summing the peaks with respect to absolute values complicates the circuit since the signal processing is performed by taking the absolute values of two signals in the I and Q channels, obtaining the peaks of the absolute values, and then summing the peaks to obtain the signal value. Moreover, if the condition of one channel is poor during the summing of the signals of the I and Q channels, an abnormal signal appears in the resultant sum. Meanwhile, since the costas loop system obtains the signal value from the output of one signal channel, the output becomes abnormal in case of an irregular loop or poor channel conditions, and more particularly, there is no output when the phase is delayed.